1. Field of the Invention
As the conventional magnetic circuit, a column center pole and a ring permanent magnet are placed on a disc yoke and a ring plate is placed on the permanent magnet so as to form a magnetic gap between the plate and the center pole. The voice coil is placed in the magnetic gap. In usual, the center pole and the plate are made of soft iron and are magnetized by the magnetic field of the permanent magnet.
When AC signal current is passed through the voice coil, the AC magnetic field is produced and the magnetic materials are magnetized by both the AC magnetic field and the DC magnetic field. This state of magnetization forms a minor hysteresis loop. The minor hysteresis loop is non-linear because of the non-linearity of the permeability, .mu. of the soft iron as the magnetic material. Accordingly, even though sinusoidal AC voltage is applied to the voice coil, harmonic components are formed in the current whereby harmonic distortion is caused in the reproduced sound in the speaker drive system.
In order to overcome these disadvantages, various improvements have been proposed. For examples, (1) a thin plate made of magnetic material is laminated on at least one facing part of the yoke (plate) and the center pole, parallel to the direction of the DC magnetic flux for excitation (Japanese Unexamined Patent Publication No. 22117/1974) and (2) a magnetic material (ferro-nickel alloy) having the rectangular hysteresis magnetic property which is not saturated by magnetic flux density in the gap is used (Japanese Unexamined Patent Publication No. 17011/1977).
There has been some success in reducing distortion by these improvements, however it has not been entirely satisfactory. The result of the various studies on the reduction of harmonic distortion has been published in Mitsubishi Electric ADVANCE Vol. 51 No. 12, 1977 page 789-792, "Reducing Harmonic Distortion in Speaker Drive Systems."